Sen. Ted Cruz (R-TX) was having the best GOP debate of the campaign–arguably, of any candidate–on Thursday night.

But an unforced error on “New York values” gave frontrunner Donald J. Trump the chance to take control.

Cruz has been using the “New York values” line against Trump as a way to respond to what he calls the “Birther” attacks on his eligibility as a “natural born citizen.” He talked about it during interviews with Howie Carr and with Megan Kelly, and provoked a response from Trump, who told Bloomberg News Wednesday that New York City “is an amazing place with amazing people,” and talked about the city’s response to the terror attacks of Sep. 11, 2001.

Astonishingly, when asked by Fox Business Network moderator Maria Bartiromo to clarify what he meant, Cruz stuck to his new catchphrase, characterizing New York as a liberal haven from which many other Americans felt alienated.

It was reminiscent of Dan Quayle’s attack on the “cultural elite” in 1992 or Sarah Palin’s characterization of more “pro-America” parts of the country in 2008. And–crucially–Cruz declined to acknowledge 9/11. Trump then pounced.

BARROOM: Senator, let me follow up and switch gears. Senator Cruz, you suggested Mr. Trump, quote, “embodies New York values.” Could you explain what you mean by that?

CRUZ: Yea, I think most people know exactly what New York values are. (Laughter)

BARTIROMO: I am from New York.

CRUZ: You’re from New York, so you might not. But I promise you, in the State of South Carolina, they do.

(Laughter, Applause)

And listen, there are many, many wonderful, wonderful working men and women in the State of New York. But everyone understands that the values in New York City are socially liberal or pro-abortion or pro-gay marriage–focus around money and the media. And I would note indeed, the reason I said that is I was asked, my friend Donald has taken to it at events playing Bruce Springsteen’s “Born in the USA.” And I was asked what I thought of that. And I said if he wanted to play a song, maybe he could play “New York, New York.” And, you know, the concept of New York values is not that complicated to figure out. Not too many years ago, Donald did a long interview with Tim Russert. And in that interview he explained his views on a whole host of issues that were very, very different from the views he’s describing now. And his explanation–he said, look, I’m from New York, that’s what we believe in New York. Those aren’t Iowa values, but this is what we believe in New York. And so that was his explanation, and I guess I can frame it another way. Not a lot of conservatives come out of Manhattan. I’m just saying.

(Laughter)

BARTIROMO: Are you sure about that?

TRUMP: Maria, some conservatives actually do come out of Manhattan. Including William F. Buckley and others, just so you understand.

(Applause)

And just so—if i could, because he insulted a lot of people–I’ve had more calls on that statement that Ted made. New York is a great place. It’s got great people, it’s got loving people, wonderful people. When the World Trade Center came down, I saw something that no place on earth could have handled more beautifully, more humanely than New York. You had two one hundred–

(Applause)

–you had two 110-story buildings come crashing down. I saw them come down. Thousands of people killed, and the cleanup started the next day, and it was the most horrific cleanup probably in the history of doing this, and in construction, I was down there. And I’ve never seen anything like it. And the people in New York fought and fought and fought, and we saw more death, and even the smell of death–nobody understood it. And it was with us for months. The smell. The air. And we rebuilt downtown Manhattan, and everybody in the world watched, and everybody in the world loved New York and loved New Yorkers, and I have to tell you, that was a very insulting statement that Ted made.

Trump’s answer was so devastating that even Cruz began applauding it midway through, hoping to climb back on the right side.

But he never regained his momentum. What had looked like a huge Cruz win went to Trump instead.